The reaction of 4-nitrotoluene with chlorine in the presence of Lewis acid catalysts such as FeCl.sub.3 (U.S. Pat. No. 3,341,595; J. Chem. Soc. 1927, 2905) or SbCl.sub.3 (Bull. Soc. Chim. Belg. 61 (1952) 317) is known. The main product formed is the desired 2-chloro-4-nitrotoluene. This result can also be explained using the generally applicable substituent rules for electrophilic aromatic substitution. The textbooks Streitwieser, Jr. and Heathcock "Introduction to Organic Chemistry" 1985, 669-670 (MacMillan Publishing Co.) and McMurry "Organic Chemistry", 1985, 506-507 (Brooks-Cole Publishing Co.), for example, show that the effect of the methyl group and the para-nitro group is additive, and that the most preferred position for the entry of the third substituent is the ortho position to the methyl group. However, as by-products, there are also observed more highly chlorinated, positionally isomeric dichloro-4-nitrotoluenes and 4-nitrobenzyl chloride, and the ring-chlorinated derivatives derived therefrom.
However, it is not possible to gain any understanding of the stepwise selectivity in the electrophilic aromatic substitution from the general substitution rules mentioned. In the present case of the ring chlorination of 4-nitroalkylbenzenes, stepwise selectivity is understood as how big the proportion of the desired monochlorinated product is in the reaction mixture, based on the conversion of 4-nitroalkylbenzene. In general, such chlorinations proceed with high selectivity at the beginning of the reaction, when virtually pure starting material is present. Thus, in accordance with the substituent rules mentioned, 2-chloro-4-nitroalkylbenzene is formed almost exclusively. However, in the course of the chlorination, the 4-nitroalkylbenzene content of the reaction mixture is being reduced, and simultaneously its 3-chloro-4-nitroalkylbenzene content increases. This additionally results in the desired product being chlorinated as well, reacting to give useless dichloronitroalkylbenzenes.
Our own studies on the ring chlorination of 4-nitrotoluene using FeCl.sub.3 as catalyst have shown that the stepwise selectivity is still insufficient. The maximum proportion of 2-chloro-4-nitrotoluene which can be achieved in the reaction mixture is just below 90%. This result correlates well with the data from RO 76842 (CA 100: 8516 (1984)). This publication states a 2-chloro4-nitrotoluene content of 86 to 89% for chlorination mixtures of 4-nitrotoluene.
Furthermore, it is known that the stepwise selectivity of the chlorination of 4-nitroalkylbenzenes can be increased by additional co-catalysts.
Small amounts of iodine, for example, can be used as co-catalyst (Naturwiss. 17 (1929) 13, Houben-Weyl "Methoden der organischen Chemie" [Methods of organic chemistry] Volume V/3 (1962), 704, JP-B-75/7589 (CA 83:113927 (1975), CS 193662 (1984)). In addition, U.S. Pat. No. 3,005,031 describes that moist 4-nitrotoluene can be chlorinated in the presence of iron, iodine and PCl.sub.3. Our own studies on the effect of iodine on the stepwise selectivity have shown, that a content of up to 95% of 2-chloro-4-nitrotoluene can be achieved in the reaction mixture under these conditions.
However, the process with addition of iodine has considerable disadvantages, rendering it unsuitable for industrial application. In particular, virtually all of the iodine remains in the chlorination mixture, even after repeated washing with water, so that even after catalytic hydrogenation and distillation, a 3-chloro-4-alkylaniline which is contaminated with iodine is obtained. Such a contaminated material cannot, for example, be used for phosgenation to 3-chloro4-alkylphenyl isocyanate.
German Offenlegungsschrift 31 28 442, too, discloses the chlorination of 4-nitrotoluene using iodine as the only catalyst. The chlorination is carried out at a temperature between the melting point of 4-nitrotoluene and 120.degree. C. 0.1 to 10% by weight of iodine, based on 4-nitrotoluene, are employed. This process likewise affords chlorination mixtures with a high content of iodine, which are unsuitable for use in practice. Moreover, in this process a large fraction of the chlorine that is introduced escapes from the reaction mixture without being utilized, so that, for example, to achieve an industrially required conversion of more than 90 mol % of chlorine, up to 300 mol % of chlorine have to be introduced per mole of 4-nitrotoluene.
EP-A-0 399 293, too, describes the ring monochlorination of 2-chloro4-nitroalkylbenzenes. The co-catalysts used here are heterocyclic 5- or 6-membered dibenzo-fused compounds which contain at least one sulphur atom. The dibenzo-fused sulphur heterocycles used are, for example, compounds from the classes of the phenoxathiins, the thianthrenes, the thianthrene 5-oxides, the thianthrene 5,5-dioxides or the dibenzothiophenes. By this route, it is possible to obtain good selectivities. Contents of, for example, 95 to 96% of 2-chloro4-nitrotoluene in the chlorination mixture are achieved. However, this process, too, has considerable disadvantages which do not permit use in large-scale industrial practice. For example, the co-catalysts have to be synthesized via relatively complicated processes, since they are not commercially available. Furthermore, there are considerable objections from a toxicological point of view against the compounds from the classes mentioned.
Accordingly, it was an object of the present invention to provide an industrially applicable process for the ring chlorination of 4-nitroalkylbenzenes which has an increased stepwise selectivity. This process should ensure, in particular at high degrees of chlorination, when the proportion of 4-nitroalkylbenzene in the reaction mixture is already very low, that the proportion of more highly chlorinated products formed is low. Such a desired increase in the stepwise selectivity is equivalent to an increase in the yield of the target product 2-chloro-4-nitroalkylbenzene.